1. Field of the Invention
This invention relates to hermetic sealing of encapsulated microelectronic devices and in particular to a thin film sealband for sealing a ceramic substrate to a cap in a multi-chip module.
2. Description of Related Art
Hermetic sealing of single-chip modules (SCM) and multi-chip modules (MCM) has conventionally been made either by mechanical sealing or by thick metallic film sealbands which typically utilize high temperature, gold based solders. MCM-D modules are produced with thin film underlying wiring for the chips deposited on the substrate surface, for example, by alternately depositing thin film layers of organic dielectric material such as polyimides and thin film metal, wiring layers such as copper. For MCM-D modules, mechanical C-ring seals have been utilized in the past. The C-ring seal typically increases module cost due to the extra hardware required to support the substrate of the module as well as the additional substrate processing to create the flange for sealing.
Hermetically sealed multi-chip modules made with ceramic based pastes to create the wiring layer (MCM-C modules) have utilized a thick (greater than 0.001 in.) film metal sealband and solder to hermetically seal the module and ceramic cap. To make the metal sealband, one or more layers of thick film metal paste may be screened onto the top layer of the ceramic MCM-C substrate and then sintered at high temperatures, generally above 500.degree. C. It has been found that use of this thick film technology is not compatible with MCM-D module processing. The thick film seaband has a significant height which interferes with thin film processing of the wiring layers beneath the semiconductor chip. Also, the MCM-D substrate must be lapped and polished prior to thin film processing to create a smooth, planar surface. This process step would necessarily remove the thick film sealband.
Furthermore, prior art methods have not provided multiple rework capability for sealbands on MCM-C modules, i.e., they have generally not permitted the solder seal to be remelted and replaced in a course of modifying or repairing a module.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an improved system and method of sealing for multi-chip modules employing thin film wiring layers for connecting the electronic chips thereon.
It is another object of the present invention to provide an improved system and method of sealing a module chip to a ceramic substrate which does not employ thick film technology.
A further object of the invention is to provide a method of sealing a module while maintaining a planar, smooth surface substrate during fabrication of thin film interconnect structures.
It is yet another object of the present invention to provide a low temperature fabrication process for module sealbands.
It is another object of the present invention to provide a hermetic solder seal for modules with multiple rework capability.
It is a further object of the present invention to provide a method of fabricating a sealband on a module substrate and subsequently seal the module after thin film interconnects have been built and tested, without affecting the performance or reliability of the thin film interconnect structure.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.